This invention relates to a method and apparatus for unloading bulk material such as fly ash from a vessel such as a storage silo while substantially preventing that material from dusting. More particularly, the invention relates to a fly ash conditioning apparatus or "dustless" unloader which is used in removing fly ash from a storage silo and emptying the fly ash into a truck or other disposal system for transit to a dump or other storage or use point. The material is wetted by applying water in a rotating drum ash conditioner. The present invention provides a method and apparatus for controlling the feed rate of material from the silo into the ash conditioner and controlling the quantity of water sprayed onto the material as a function of the rate of flow of material from the silo into the conditioner.
Prior to the present invention, ash conditioning systems utilizing water sprayed onto the ash are generally known. Ash conditioners of this type include a drum mounted for rotation about its own axis and mounted on an incline. The conditioner includes an inlet at its upper end for receiving dry ash discharged from the storage silo and a wetted ash outlet at the lower end of the conditioner. The conditioner includes an internal arrangement for spraying water onto the surface of material within the drum. As the drum is rotated, the ash tumbles down the length of the ash conditioner from its inlet to its outlet and the water spary wets substantially all of the material. The wetted material is then dumped into a truck or train hopper for transit to a dump or other use point. Due to the wetting of material, dusting of the material during the dump into the truck and transit in the truck is substantially reduced.
One of the major problems with prior ash conditioning systems is that the material may be over or under wetted. If the ash is wetted an inadequate amount, the intended purpose of reduction in dusting will not be achieved. If the ash is wetted too much, drainage and housekeeping problems result. Excess wetting of ash from western coal is a more severe problem than with ash from other coals in that the Western ash may contain materials which, if wetted above about 12% moisture, the ash becomes cementatious and will cling to virtually any surface, particularly inside the ash conditioner and associated equipment and the transport equipment. It is known that if ash from western coal becomes excessively wetted, it will set up in the ash conditioner and other places to such an extent that it must be removed by pneumatic hammers. At the same time, if the material is wetted to less than about 8% moisture, excessive dusting will occur during material handling and the purpose of using the ash conditioner will not be served. If is therefore, important to maintain the moisture content of Western coal ash between 8% and 12%.
Prior to the present invention, typically, the ash conditioner was close coupled to the outlet of the storage silo so that material is directly discharged from the silo into the ash wetting device. Water is supplied to the device at a steady rate. Fly ash will tend to set up in silos so that material discharge is not at a steady rate. Bridging of material across the walls of the silo can occur which results in material flow though the silo outlet being stopped. With a steady flow of water to the ash conditioner, a stoppage of material flow results in excessive wetting of material that is in the ash conditioner. When the bridge formed in the silo breaks, there will be a flush of material through the silo outlet into the ash conditioner. A steady flow of water into the conditioner will be insufficient to adequately wet the fly ash.
It is to be understood that method and apparatus of the present invention will not totally eliminate dusting of the fly ash when it is unloaded from the silo into the vessel. However, with a proper quantity of water applied to the fly ash, dusting will be substantially prevented even though some material dust is likely to still be present.